What about a 20" X 28" frame with a 16" X 24" (3:2) print? That's not poster-size if you're using a mat board cut-out and giving you a 2" border.

It is all about framing aesthetics. As with photography, there are some guidelines that tend to be useful. Generally, the larger the print, the larger the border To me, a print of the above size simply looks lousy framed with such a small border. Of course a killer print overcomes lesser issues of presentation for the most part.

To name a few more:

8X10 frame for a 4X6 print giving a 2" border.

To me that looks lousy too.

11X14 frame for a 6X9 print giving a 2.5" border.

That looks OK.

Why don't you like to crop your shots out of a 3:2 constrained space? Shouldn't the image (or part of therein) determine the crop and the crop ratio employed for aesthetic objectives?

Yes we are in agreement. When I shoot, I'm very mindful of the viewfinder frame. I'm not dogmatic about cropping, and will go square when it looks better. When I have shot in 645 or 4x5 formats I like to stay with those aspect ratios as well. I'm more interested in image content than the wrapper.

But my work style has integrated the 3:2 aspect ratio and it has served me well.

It's just my opinion but sticking to a 3:2 ratio can detrimentally constrain photography.

My opinion is that any aspect ratio constrains the frame out of necessity and it has little to do with photography.